Pump apparatus

ABSTRACT

A constant rate discharge pump comprises a rotary shaft which is rotatable together with a rotary driving source, a piston which is displaceable in an axial direction in a pump chamber of a body by the rotation of the rotary shaft and which has a tapered surface having diameters reduced downwardly on an outer circumference thereof, and a skirt section which is disposed on the piston and which extends radially outwardly. The constant rate discharge pump further includes a valve plug membrane of a resin material which is displaceable together with the piston, and a pressure sensor installed in the body which detects a pressure of a fluid flowing through the pump chamber.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a pump apparatus which makes itpossible to discharge a constant amount of a fluid all the time bycontrolling the flow rate of the fluid by rotation of a driving source.

[0003] 2. Description of the Related Art

[0004] A constant rate discharge pump has been adopted in order tosupply a constant amount of a chemical solution, a paint, a washingsolution, or the like, for an apparatus of producing semiconductors orthe like, a painting apparatus, and a medical apparatus.

[0005] A bellows type pump is often used for the constant rate dischargepump. In the bellows type pump, the suction pressure and the dischargepressure are obtained by expanding/contracting bellows surrounding ashaft member, driven by a motor or the like.

[0006] In this apparatus, the shaft member is displaced in the axialdirection by the driving source such as the motor. The tip section ofthe shaft member is displaced in a pump chamber which is formed in apump housing. The bellows is interposed between the tip section and thepump chamber, and the bellows is expanded/contracted when the tipsection is displaced. The suction pressure is generated when the bellowsis contracted in the pump chamber. Accordingly, liquid is sucked fromthe outside, and the pump chamber is filled with the liquid. On theother hand, the discharge pressure is generated by expanding the bellowsin the pump chamber. Accordingly, the liquid is discharged from the pumpchamber to the outside (see, for example, Japanese Laid-Open PatentPublication No. 10-47234).

[0007] In the case of the conventional constant rate discharge pump,when the flow rate of the fluid to be sucked and discharged isincreased, it is necessary to set a large stroke of the shaft member andthe tip section in the axial direction in response to the flow rate. Insuch a situation, the bellows needs to be large, which isexpanded/contracted in conformity with the increase of the strokeamount. However the production cost becomes expensive, because thebellows is expensive.

[0008] When the flow rate of the fluid to be sucked and discharged isincreased, the amounts of expansion and contraction of the bellows areincreased. As a result, some pulsation may occur in the fluid when thefluid is discharged from the pump chamber to the outside.

[0009] Further, when a liquid is sucked into the pump chamber, theliquid remaining in the pump chamber after discharging the liquid fromthe pump chamber to the outside may be pooled on the outercircumferential surface of the bellows.

SUMMARY OF THE INVENTION

[0010] A general object of the present invention is to provide a pumpapparatus which makes it possible to reduce the cost and which makes itpossible to discharge a constant amount of a fluid highly accuratelywithout causing any pulsation of the fluid even when the large amount offluid flows in the pump.

[0011] The above and other objects, features, and advantages of thepresent invention will become more apparent from the followingdescription when taken in conjunction with the accompanying drawings inwhich a preferred embodiment of the present invention is shown by way ofillustrative example.

BRIEF DESCRIPTION OF THE DRAWINGS

[0012]FIG. 1 is a vertical sectional view taken in the axial directionillustrating a constant rate discharge pump according to an embodimentof the present invention;

[0013]FIG. 2 is a vertical sectional view taken in the axial directionillustrating a state in which a piston is displaced in the direction ofthe arrow X1 starting from a state shown in FIG. 1;

[0014]FIG. 3 is a lateral sectional view taken along a line III-IIIshown in FIG. 1;

[0015]FIG. 4 is, with partial omission, a magnified vertical sectionalview taken in the axial direction illustrating the displacement in theaxial direction of a valve plug membrane of the constant rate dischargepump shown in FIG. 1;

[0016]FIG. 5 is, with partial omission, a vertical sectional view takenin the axial direction illustrating a constant rate discharge pumpaccording to another embodiment of the present invention; and

[0017]FIG. 6 is, with partial omission, a vertical sectional view takenin the axial direction illustrating a state in which a piston isdisplaced in the direction of the arrow X1 starting from a state shownin FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] With reference to FIG. 1, reference numeral 10 indicates aconstant rate discharge pump according to an embodiment of the presentinvention.

[0019] The constant rate discharge pump 10 comprises a body 12 in whichfluid passages 24 a, 24 b for flowing the fluid are formed, first andsecond joint members 14, 15 which are connected to side portions of thebody 12 and to which unillustrated tubes are detachably connected, abonnet 16 which is connected to an upper portion of the body 12, and adriving section 20 which is provided in a cover member 18 arranged onthe bonnet 16 and which is driven and rotated by an electric signalsupplied from an unillustrated power source. The constant rate dischargepump 10 further comprises a holding member 22 which is interposedbetween the bonnet 16 and the driving section 20 for holding a bearing92 as described later on, and a flow rate control mechanism 26 whichcontrols the flow rate of the fluid flowing through the fluid passages24 a, 24 b by the driving section 20.

[0020] A pump chamber 29 is provided at a substantially central portionof the body 12 under the lower surface of a valve plug membrane 28 of aresin material which is formed flexibly or bendably. A seat section 30is formed at a lower portion of the pump chamber 29, on which the valveplug membrane 28 is seated. The seat section 30 has a tapered shape withdiameters decreased gradually downwardly.

[0021] A through-hole 32 is formed in the axial direction in the body12, and communicates with the pump chamber 29 via the seat section 30. Apressure sensor 36 is installed into the through-hole 32 by an adapter34.

[0022] A detecting section 38 is provided at an upper portion of thepressure sensor 36 to detect the pressure of the fluid flowing into thepump chamber 29. The pressure sensor 36 is connected to an unillustratedcontroller via a lead wire 40. The pressure value detected by thedetecting section 38 is outputted as an output signal to the controller.

[0023] A plug 42 is screwed with and closes the through-hole 32 upwardlywhile the pressure sensor 36 is installed to the through-hole 32. Thelead wire 40 of the pressure sensor 36 is guided to the outside via ahole formed through a substantially central portion of the plug 42.

[0024] On the other hand, the fluid passages 24 a, 24 b are formedthrough the side portions of the body 12. The fluid passage 24 acommunicates with the pump chamber 29 of the body 12 and first port 54of the first joint member 14. The fluid passage 24 b communicates withthe pump chamber 29 of the body 12 and second port 56 of the secondjoint member 15. That is, the fluid passage 24 a is formed near thefirst joint member 14, and the fluid passage 24 b is formed near thesecond joint member 15.

[0025] A large diameter section 46 b is formed in the fluid passage 24 bnear the second joint member 15. The large diameter section 46 b hasexpanded diameters radially outwardly as compared with the innerdiameter of the second port 56. A spherical valve plug 48 b is arrangedin the large diameter section 46 b, which functions as a second checkvalve 47 b. The valve plug 48 b has a diameter which is slightly largerthan the inner diameter of the fluid passage 24 b. A valve seat section50 b is formed on the large diameter section 46 b. The valve seatsection 50 b has a tapered shape (see FIG. 2) with its diametersgradually reduced toward the fluid passage 24 b.

[0026] A spring (second spring) 52 b is interposed between the valveplug 48 b and a connecting member 60 b installed to the second jointmember 15 (as described later on). The spring 52 b urges the valve plug48 b in the direction in which the valve plug 48 b is pressed againstthe valve seat section 50 b. That is, the valve plug 48 b is seated onthe valve seat section 50 b by being pressed under the action of thespring force of the spring 52 b. Accordingly, the communication betweenthe fluid passage 24 b and the large diameter section 46 b is shut offby the valve plug 48 b.

[0027] The first joint member 14 and the second joint member 15 areconnected to the side portions of the body 12, so that the first jointmember 14, the second joint member 15, and the body 12 are aligned. Thefluid is sucked through the first joint member 14 from the outside viathe unillustrated tube, and the fluid is discharged through the secondjoint member 15 to the outside via the tube.

[0028] The first port 54 is formed in the first joint member 14, and thesecond port 56 is formed in the second joint member 15. The first andsecond ports 54, 56 communicate with the fluid passages 24 a, 24 b ofthe body 12, respectively, via the connecting members 60 a, 60 b.

[0029] The connecting members 60 a, 60 b are arranged in installationholes disposed at the ends of the first and second ports 54, 56 near thebody 12, respectively. The connecting members 60 a, 60 b are interposedbetween the body 12 and the first and second joint members 14, 15,respectively.

[0030] Communication passages 62 a, 62 b are formed penetratingly atsubstantially central portions of the connecting members 60 a, 60 b. Thefirst and second ports 54, 56 communicate with the fluid passages 24 a,24 b via the communication passages 62 a, 62 b, respectively.

[0031] Inner members 64 are engaged with the first port 54 of the firstjoint member 14 and the second port 56 of the second joint member 15,respectively. Lock nuts 66 are screwed with the ends of the first andsecond joint members 14, 15 while the openings of the unillustratedtubes are inserted into the inner members 64. Accordingly, the liquidtightness is retained at the connecting portions of the tubes when thelock nuts 66 are screwed.

[0032] On the other hand, a large diameter section 46 a is formed nearthe body 12 in the first port 54. The large diameter section 46 a isdiametrally expanded radially outwardly as compared with the innerdiameter of the first port 54. A spherical valve plug 48 a is arrangedin the large diameter section 46 a and functions as a first check valve47 a. The valve plug 48 a has a diameter which is slightly larger thanthe inner diameter of the first port 54.

[0033] A valve seat section 50 a is formed at the end of the largediameter section 46 a. The valve seat section 50 a has a tapered shapewith its diameters gradually reduced toward the first port 54.

[0034] A spring (first spring) 52 a is interposed between the valve plug48 a and a connecting member 60 a. The spring 52 a urges the valve plug48 a in the direction in which the valve plug 48 a is pressed againstthe valve seat section 50 a. That is, the valve plug 48 a is seated onthe valve seat section 50 a while pressed by the spring force of thespring 52 a. Accordingly, the communication between the first port 54and the large diameter section 46 a is shut off by the valve plug 48 a.

[0035] The driving section 20 includes a rotary driving source 70 whichis rotatable in accordance with an electric signal supplied from anunillustrated power source, and a drive shaft 72 which transmits therotary driving force of the rotary driving source 70.

[0036] The rotary driving source 70 is, for example, a step motor. Therotary driving source 70 is arranged on the upper surface of a platemember 74 in the cover member 18. The drive shaft 72 penetrates throughthe plate member 74 and protrudes from the lower surface of the rotarydriving source 70. The drive shaft 72 is rotated together with therotation of the rotary driving source 70.

[0037] A connecting member 76 having a substantially C-shaped crosssection is inserted upwardly into the lower end of the drive shaft 72.The connecting member 76 is integrally installed to the drive shaft 72by a screw member 78 which is screwed in the direction substantiallyperpendicular to the axis of the drive shaft 72 from the outercircumferential surface thereof.

[0038] Engaging pins 82 are installed to a plurality of grooves formedon the outer circumferential surface of the connecting member 76 so thatthe engaging pins 82 protrude radially outwardly. The engaging pins 82are provided at two positions so that the engaging pins 82 are spacedfrom each other by a predetermined angle in the circumferentialdirection of the connecting member 76.

[0039] The flow rate control mechanism 26 includes a rotary shaft 84which is rotatable together with the rotation of the rotary drivingsource 70, a piston 86 which is displaceable in the axial direction inthe bonnet 16 by the rotation of the rotary shaft 84, and the valve plugmembrane 28 which is integrally connected to the piston 86.

[0040] The rotary shaft 84 is elongate, and is arranged under theconnecting member 76.

[0041] A disk-shaped flange section 88 diametrally expanded outwardly isformed at an upper portion of the rotary shaft 84. The flange section 88is interposed between the bearing 92 and a spacer 90. The spacer 90 isinterposed between the holding member 22 and the bonnet 16. Accordingly,the displacement of the rotary shaft 84 in the axial direction isrestricted.

[0042] An annular projection 94 protruding upwardly by a predeterminedlength is formed on the upper surface of the flange section 88. Theouter circumferential surface of the projection 94 is rotatablysupported by the bearing 92. Grooves are formed at positions opposed tothe engaging pins 82 of the connecting member 76 on the innercircumferential side of the projection 94. Each of the grooves isrecessed by a predetermined length. The engaging pins 82 are engagedwith the grooves.

[0043] That is, the engaging pins 82, which are engaged with theconnecting member 76, are engaged with the grooves of the rotary shaft84. Thus, the rotary shaft 84 is rotated together with the rotation ofthe rotary driving source 70 via the connecting member 76.

[0044] On the other hand, a screw section 98 is formed at a lowerportion of the rotary shaft 84, on which a screw is engraved on theouter circumferential surface. The screw section 98 is screwed with ascrew hole 101 of the piston 86 which is provided displaceably in theaxial direction in the bonnet 16.

[0045] The piston 86 of the resin material is displaced in the axialdirection by the rotation of the rotary shaft 84, and the outercircumferential surface of the piston 86 slides along the inner wallsurface 99 of the bonnet 16.

[0046] A pair of rotation-preventive pins 100 are installed to groovesformed on the outer circumferential surface of the piston 86, andprotrude radially outwardly by predetermined lengths. Therotation-preventive pins 100 are engaged with a pair of engaging grooves102 which are formed and recessed by predetermined lengths on the innerwall surface 99 of the bonnet 16 (see FIG. 3). Each of the engaginggrooves 102 is substantially linear in the axial direction. That is,when the piston 86 is displaced in the axial direction by the rotarydriving source 70, the rotation-preventive pins 100 are engaged with theengaging grooves 102. Therefore, the rotation of the piston 86 in thecircumferential direction is prevented.

[0047] Wear rings 104 are installed to annular grooves formed on theouter circumferential surface of the piston 86. Further, a taperedsurface 106 (see FIG. 4) is formed on the outer circumferential surfaceof the piston 86, which is inclined by a predetermined angle so that thediameters are gradually reduced downwardly from the portions of theouter circumferential surface of the piston 86 at which the wear rings104 are installed. A chamfered section 106 a as shown in FIG. 4 isformed at the lower end of the tapered surface 106.

[0048] A screw hole 108 is formed in the axial direction in the piston86. A shaft section 110 of the valve plug membrane 28 of the resinmaterial is integrally screwed with the screw hole 108 as describedlater on. That is, the valve plug membrane 28 is displaced together withthe displacement of the piston 86 in the axial direction. A hole 112,which is open upwardly, is formed in the shaft section 110 of the valveplug membrane 28. When the valve plug membrane 28 is displaced upwardly,the screw section 98 of the rotary shaft 84 is inserted thereinto.Therefore, the hole 112 has a diameter which is slightly larger than thediameter of the screw section 98 of the rotary shaft 84.

[0049] The valve plug membrane 28 is formed of the resin material suchas PTFE (polytetrafluoroethylene), which is a fluororesin. The valveplug membrane 28 includes the shaft section 110 which is screwed intothe piston 86, a thick-walled main valve body section 114 which isformed under the shaft section 110 and which is diametrally expandedoutwardly as compared with the shaft section 110, and a skirt section116 which extends radially outwardly from the upper surface of the mainvalve body section 114. A circumferential edge 118 of the skirt section116 of the valve plug membrane 28 is fitted into and supported in anannular recess 120 which is formed by the body 12 and the bonnet 16.

[0050] The skirt section 116 is connected to the upper circumferentialedge of the main valve body section 114, which is formed to rise orstands in conformity with or along the tapered surface 106 of the piston86. On the other hand, the skirt section 116 is connected to the upperportion of the circumferential edge 118 to rise or stands in conformitywith or along the inner wall surface 99 of the bonnet 16 (see FIGS. 1and 2).

[0051] The lower surface of the main valve body section 114 has atapered shape with diameters gradually reduced downwardly correspondingto the seat section 30 of the body 12. When the piston 86 is displacedto the lower end, the lower surface of the main valve body section 114abuts against the seat section 30 of the body 12 tightly.

[0052] The skirt section 116 is formed as a bendable thin-walledmembrane. When the piston 86 is displaced downwardly, the skirt section116 is gradually disposed on or engaged with the tapered surface 106 ofthe piston 86 from the vicinity of the main valve body section 114radially outwardly. Also, the portion of the skirt section 116 in thevicinity of the circumferential edge 118 is bent or curved to be convexupwardly between the main valve body section 114 and the inner wallsurface 99 of the bonnet 16 (see FIGS. 1 and 4).

[0053] On the other hand, when the piston 86 is displaced upwardly, theskirt section 116 is gradually disposed on or engaged with the innerwall surface 99 of the bonnet 16 radially inwardly from the vicinity ofthe circumferential edge 118, and the portion of the skirt section 116in the vicinity of the main valve body section 114 is bent or curved tobe convex upwardly between the main valve body section 114 and the innerwall surface 99 of the bonnet 16 (see FIGS. 2 and 4).

[0054] As for the valve plug membrane 28, the lower surface of the mainvalve body section 114 abuts against the seat section 30 of the body 12,when the piston 86 is displaced to the lower end by the rotation of therotary driving source 70. Accordingly, the communication is shut offbetween the fluid passage 24 a near the first port 54 and the fluidpassage 24 b near the second port 56.

[0055] The constant rate discharge pump 10 according to the embodimentof the present invention is basically constructed as described above.Next, its operation, function, and effect will be explained. Theexplanation will be made assuming that the initial state is as shown inFIG. 1, in which the main valve body section 114 of the valve plugmembrane 28 connected to the piston 86 contacts the seat section 30 ofthe body 12.

[0056] Firstly, for example, an unillustrated coating liquid supplysource for semiconductor is connected to the first port 54 of the firstjoint member 14 via an unillustrated tube. On the other hand, forexample, an unillustrated coating liquid-dripping apparatus is connectedto the second port 56 of the second joint member 15 via an unillustratedtube.

[0057] Subsequently, a driving signal is outputted from theunillustrated controller to the rotary driving source 70 on the basis ofthe preset flow rate of the fluid with the controller.

[0058] The current is supplied to the rotary driving source 70 from theunillustrated power source, the drive shaft 72 is rotated by therotation of the rotary driving source 70, and the rotary shaft 84 isrotated together with the drive shaft 72. In this situation, the rotaryshaft 84 is not displaced in the axial direction by the rotation,because the flange section 88 of the rotary shaft 84 is interposedbetween the bearing 92 and the spacer 90.

[0059] As shown in FIG. 2, the piston 86 screwed with the screw section98 is displaced upwardly (in the direction of the arrow X1) underscrewing relationships of the piston 86 in accordance with the rotationof the rotary shaft 84. Accordingly, the interior of the pump chamber 29closed by the valve plug membrane 28 connected to the piston 86 is in asuction state (negative pressure state).

[0060] When the interior of the pump chamber 29 is in the negativepressure state, the valve plug 48 a, which is installed in the firstjoint member 14, is separated from the valve seat section 50 a againstthe spring force of the spring 52 a, and the valve plug 48 a isdisplaced toward the body 12.

[0061] As a result, the first port 54 of the first joint member 14communicates with the fluid passage 24 a of the body 12. The fluid (forexample, the coating liquid) passes through the tube connected to theunillustrated coating liquid supply source for semiconductor, and thefluid is supplied from the first port 54 into the pump chamber 29 viathe communication passage 62 a of the connecting member 60 a and thefluid passage 24 a.

[0062] The valve plug 48 a, which is arranged in the first joint member14, functions as the first check valve 47 a such that the valve plug 48a is seated on the valve seat section 50 a in accordance with the springforce of the spring 52 a.

[0063] Accordingly, when the fluid, which has been supplied into thepump chamber 29 of the body 12, is about to cause counterflow toward thefirst port 54, the fluid is prevented from the counterflow by the valveplug 48 a seated on the valve seat section 50 a.

[0064] When the piston 86 is displaced to a position which is based onthe flow rate of the fluid previously set by the controller, then a stopsignal is outputted from the controller to the rotary driving source 70,and the supply of the current is stopped. As the rotary driving source70 is stopped, the displacement of the piston 86 in the axial directionis stopped. That is, the flow rate of the fluid sucked into the pumpchamber 29 is established by the upward displacement amount in the axialdirection from the initial position at which the valve plug membrane 29is seated on the seat section 30.

[0065] When the piston 86 is displaced in the axial direction, thepiston 86 is prevented from any rotation, because therotation-preventive pins 100, which are installed to the outercircumference of the piston 86, are engaged with the engaging grooves102 (see FIG. 3).

[0066] In this situation, the upper surface of the skirt section 116 ofthe valve plug membrane 28 is disposed on or engaged with the inner wallsurface 99 of the bonnet 16 from the circumferential edge 118 which isinterposed between the body 12 and the bonnet 16. The portion betweenthe main valve body section 114 and the skirt section 116 engaged withthe inner wall surface 99 is retained in a state of being bent or curvedupwardly.

[0067] That is, when the valve plug membrane 28 is displaced upwardly bythe displacement of the piston 86, the skirt section 116 is engaged withor disposed on the inner wall surface 99 of the bonnet integrally.Therefore, when the fluid is supplied into the pump chamber 29 of thebody 12, the flow of the fluid is not inhibited or blocked by the skirtsection 116 of the valve plug membrane 28 (see FIG. 4).

[0068] Next, when the characteristic of the current to be supplied tothe rotary driving source 70 is reversed from the above, the rotarydriving source 70 is rotated in the opposite direction, and thus therotary shaft 84 is rotated together with the drive shaft 72 in theopposite direction. The piston 84 is displaced downwardly (in thedirection of the arrow X2) in the axial direction under the screwingrelationships of the piston 86 with the rotary shaft 84.

[0069] When the piston 86 is displaced downwardly, the fluid containedin the pump chamber 29 is pressed by the valve plug membrane 28. Thepressed fluid urges the valve plug 48 b installed in the fluid passage24 b, the valve plug 48 b is thereby separated from the valve seatsection 50 b against the spring force of the spring 52 b, and the valveplug 48 b is displaced toward the second joint member 15. Accordingly,the interior of the pump chamber 29 communicates with the second port 56via the fluid passage 24 b. The fluid contained in the pump chamber 29is discharged via the unillustrated tube to the coating liquid-drippingapparatus connected to the second port 56. A constant amount of thefluid (for example, the coating liquid) is dripped onto thesemiconductor wafer all the time.

[0070] The valve plug 48 b, which is arranged in the large diametersection 46 b of the second joint member 15, functions as the secondcheck valve 47 b such that the valve plug 48 b is seated on the valveseat section 50 b by the spring force of the spring 52 b. Accordingly,when the fluid, which has been discharged to the outside from the secondport 56, is about to cause counterflow into the pump chamber 29 again,the fluid is prevented from the counterflow by the valve plug 48 bseated on the valve seat section 50 b.

[0071] On the other hand, when the fluid flows through the interior ofthe pump chamber 29, the pressure of the fluid flowing through theinterior of the pump chamber 29 is detected by the pressure sensor 36which is installed to the lower portion of the body 12. The detectedpressure is outputted as a detection signal to the unillustratedcontroller via the lead wire 40 of the pressure sensor 36.

[0072] The controller calculates the flow rate A of the fluid flowingthrough the pump chamber 29 on the basis of the detection signal(pressure value) supplied from the pressure sensor 36. The controllerperforms the following feedback control. The controller judges thedifference (|A−B|) between the calculated flow rate A and the presetflow rate B of the fluid previously set by the controller. Thecontroller outputs a control signal to the rotary driving source 70 sothat the difference (|A−B|) becomes zero.

[0073] As a result, the preset flow rate B of the fluid corresponds tothe amount of rotation of the rotary driving source 70. Therefore, it ispossible to flow the fluid at a preset constant flow rate into the pumpchamber 29 of the body 12. In other words, it is possible to perform thehighly accurate flow rate control of the fluid so that the flow rate ofthe fluid discharged from the second port 56 is always constant.

[0074] For example, when the flow rate A of the fluid discharged fromthe second port 56 is larger than the preset value B previously set bythe unillustrated controller (A>B), then the pressure value of the fluidis detected by the pressure sensor 36, and the detection signal(pressure value) is outputted to the controller. The controller judgesthe difference (|A−B|) between the preset value previously set by thecontroller and the flow rate of the fluid. The controller output thecontrol signal to the rotary driving source 70 so that the difference(|A−B|) becomes zero.

[0075] Subsequently, the piston 86 is displaced upwardly (in thedirection of the arrow X1) by the rotary driving source 70 on the basisof the control signal. The volume of the pump chamber 29 of the body 12is increased by the valve plug membrane 28. The pressure of the fluidflowing through the interior of the pump chamber 29 is decreased, andthe flow rate becomes the preset flow rate B (A=B).

[0076] Accordingly, the flow rate of the fluid discharged from theinterior of the pump chamber 29 to the second port 56 is decreased, andthe preset flow rate is obtained. As a result, it is possible to controlthe flow rate of the fluid highly accurately so that the flow rate ofthe fluid discharged from the second port 56 is always constant.

[0077] That is, the pressure of the fluid flowing through the interiorof the pump chamber 29 is always detected by the pressure sensor 36, andthe obtained pressure value is outputted as the detection signal to theunillustrated controller. The controller judges the difference (|A−B|)between the preset flow rate B of the fluid previously set by thecontroller and the calculated flow rate A. The control signal isoutputted to the rotary driving source 70 so that the difference (|A−B|)becomes zero.

[0078] When the rotary driving source 70 is rotated on the basis of thecontrol signal, the valve plug membrane 28 is displaced in the axialdirection together with the piston 86. As a result, the volume of thepump chamber 29 of the body 12 to be supplied with the fluid isincreased/decreased. Therefore, it is possible to control the flow rateof the fluid flowing through the pump chamber 29. Accordingly, the flowrate A of the fluid flowing through the interior of the pump chamber 29is always controlled to be substantially equivalent to the preset valueB. Thus, it is possible to always discharge a constant amount of thefluid from the second port 56.

[0079] The tapered surface 106, which has diameters reduced toward themain valve body section 114 of the valve plug membrane 28, is providedon the outer circumferential surface of the piston 86. Therefore, asshown in FIG. 1, when the piston 86 is displaced downwardly (in thedirection of the arrow X2), the upper surface of the skirt section 116is gradually engaged with or disposed on the tapered surface 106 fromthe side near the main valve body section 114. The portion between theengagement with the tapered surface 106 and the circumferential edge 118of the skirt section 116 is retained in a bent or curved state.Accordingly, the skirt section 116 of the valve plug membrane 28 of theresin material can be preferably bent along the tapered surface 106 ofthe piston 86.

[0080] As described above, in the embodiment of the present invention,when the piston 86 is displaced downwardly (in the direction of thearrow X2) by the rotary driving source 70, as shown in FIG. 4, the skirtsection 116 of the valve plug membrane 28 of the resin material can bepreferably bent while effecting the gradual engagement along the taperedsurface 106 of the piston 86. Therefore, even when the piston 86 isdisplaced downwardly, the skirt section 116 of the valve plug membrane28 does not inhibit the flow of the fluid in the pump chamber 29 of thebody 12.

[0081] The flow rate of the fluid flowing through the interior of thepump chamber 29 is controlled by integrally providing the valve plugmembrane 28 of the resin material disposed at the lower portion of thepiston 86 and displacing the valve plug membrane 28 in the axialdirection under the driving action of the rotary driving source 70. Inthis arrangement, the valve plug membrane 28, which is formed of theresin material, has the high rigidity as compared with a diaphragm orthe like which is composed of an elastic material. Therefore, the thinskirt section 116 of the valve plug membrane 28 is prevented from beingwarped.

[0082] As a result, it is possible to ensure the large stroke of thepiston 86 in the axial direction because the skirt section 116 isprevented from the warpage. It is possible to discharge fluid highlyaccurately without causing any pulsation of the fluid even when thefluid flows in a large volume through the constant rate discharge pump10.

[0083] Further, it is possible to reduce the production cost as comparedwith a product having the conventional bellows, because the valve plugmembrane 28 is formed of the resin material even when the stroke amountof the piston 86 is set to be large.

[0084] Further, even when the fluid flowing through the interior of thepump chamber 29 is a liquid, the liquid does not remain on the lowersurface of the valve plug membrane 28 after the liquid is discharged tothe outside from the pump chamber 29. Accordingly, any liquid pool onthe lower surface of the valve plug membrane 28 can be avoided.

[0085] Next, a constant rate discharge pump 150 according to anotherembodiment is shown in FIGS. 5 and 6. The constituent elements that aresame as those of the constant rate discharge pump 10 shown in FIGS. 1and 2 are designated by the same reference numerals, and any detailedexplanation thereof will be omitted.

[0086] The constant rate discharge pump 150 according to the anotherembodiment is different from the constant rate discharge pump 10according to the embodiment described above in that a plurality ofannular grooves 154, which are spaced from each other by predetermineddistances, are formed in the circumferential direction on a taperedsurface 106 of a piston 152. The annular groove 154 formed on thetapered surface 106 is not limited to any shape provided that theannular groove 154 is recessed by a predetermined depth with respect tothe tapered surface 106.

[0087] An explanation will be made about a state (see FIG. 4) in whichthe piston 152 is displaced downwardly (in the direction of the arrowX2) by the rotary driving source 70, and the skirt section 116 isengaged with or disposed on the tapered surface 106 of the piston 152,for example, as shown in FIG. 5. In this state, the contact area betweenthe tapered surface 106 and the upper surface of the skirt section 116is decreased by the annular grooves 154 as compared with the case inwhich the annular grooves 154 are not provided.

[0088] Accordingly, the sticking force of the skirt section 116 withrespect to the tapered surface 106 is decreased when the piston 152 isdisplaced downwardly (in the direction of the arrow X2). When the piston152 is displaced upwardly (in the direction of the arrow X1), the skirtsection 116 can be preferably and reliably separated from the taperedsurface 106 of the piston 152. Therefore, it is possible to displace thepiston 152 in the axial direction more smoothly.

[0089] While the invention has been particularly shown and describedwith reference to preferred embodiments, it will be understood thatvariations and modifications can be effected thereto by those skilled inthe art without departing from the spirit and scope of the invention asdefined by the appended claims.

What is claimed is:
 1. A pump apparatus comprising: a rotary drivingsource driven and rotated by an electric signal; a body provided withfluid passages for communicating with a first port for sucking a fluidand a second port for discharging said fluid; a bonnet connected to saidbody and having a pump chamber defined therein; a piston provided insaid pump chamber displaceably in an axial direction of said body undera rotary driving action of said driving source, said piston having atapered surface on an outer circumference with diameters graduallyreduced toward said fluid passages; and a valve plug membrane connectedto said piston and provided with a flexible thin skirt section extendingradially outwardly from said piston, wherein said valve plug membranehas a circumferential edge which is interposed between said body andsaid bonnet, and said valve plug membrane is displaceable in said axialdirection while being retained in a state in which said valve plugmembrane is bent convexly in one of displacement directions of saidpiston.
 2. The pump apparatus according to claim 1, wherein said skirtsection is disposed on said tapered surface of said piston, and saidskirt section rises along said tapered surface, when said piston isdisplaced in said axial direction.
 3. The pump apparatus according toclaim 1, wherein said skirt section is disposed on an inner wall surfaceof said bonnet, and said skirt section rises along said inner wallsurface, when said piston is displaced in said axial direction.
 4. Thepump apparatus according to claim 1, wherein said piston has asubstantially circular arc-shaped chamfered section which is formed at aposition facing a seat section of said body for seating said valve plugmembrane thereon.
 5. The pump apparatus according to claim 1, furthercomprising: a detecting section provided in said body for sensing apressure of said fluid flowing through said fluid passages of said body,wherein a displacement amount of said piston is controlled based on apressure value detected by said detecting section.
 6. The pump apparatusaccording to claim 1, wherein said body is provided therein with a firstcheck valve provided on said first port and pressed in a directiontoward said first port by a resilient force of a first spring, and asecond check valve provided on said second port and pressed in adirection toward said pump chamber by a resilient force of a secondspring.
 7. The pump apparatus according to claim 6, wherein said firstcheck valve is opened and said fluid is supplied into said pump chamberfrom said first port when said piston is displaced in a direction toseparate from said seat section of said body, while said second checkvalve is opened and said fluid contained in said pump chamber flows fromsaid second port to outside when said piston is displaced in a directionto approach said seat section.
 8. The pump apparatus according to claim1, wherein a rotary shaft is screwed with said piston in said axialdirection, and an end of said rotary shaft is connected to a drive shaftof said driving source through a connecting member.
 9. The pumpapparatus according to claim 8, wherein the displacement of said shaftin said axial direction is restricted, and said rotary shaft is retainedrotatably by a bearing.
 10. The pump apparatus according to claim 1,wherein an engaging groove recessed linearly in said axial direction isformed on an inner portion of said bonnet, and a rotation-preventive pinattached to said piston is engaged with said engaging groove.
 11. Thepump apparatus according to claim 10, wherein said engaging pin isdisplaced along said engaging groove when said piston is displaced insaid axial direction in said bonnet.
 12. The pump apparatus according toclaim 1, wherein grooves, each of which is recessed by a predetermineddepth, are formed on said tapered surface of said piston.
 13. The pumpapparatus according to claim 12, wherein said grooves are provided in acircumscribing manner along said tapered surface of said piston.